This invention relates generally to computer systems, particularly to microcomputer systems such as personal computers and word processors, and more particularly to such microcomputer systems of the kind including a data storage device or devices employing rotating disks such as flexible magnetic disks as storage media, and a stepper motor for incrementally driving a transducer or magnetic head unit from track to track on the disk. Still more particularly, the invention deals, in such microcomputer systems, with a method of, and means for, driving the stepper motor at high speed against the risk of seek errors that would conventionally have been liable to occur if a clock, customarily incorporated in disk drives, were powered on and off during the operation of the microcomputer system in order to save power.
The stepper motor ranks with the voice coil motor as actuator most commonly used in disk drives for moving the head unit to any desired track on the disk. Shoji et al. U.S. Pat. No. 4,594,620, assigned to the assignee of the instant application, is herein cited as teaching a head drive system with a four phase, single phase drive stepper motor.
In disk drives operating under the control of a host system, the stepper motor is controlled by stepping pulses and a stepping direction signal supplied from the host. Each external stepping pulse from the host may correspond to either one or, for higher track seek speed, two or more increments of the stepper motor. Conventionally, for driving the stepper motor at twice the recurrence rate of the external stepping pulses, for example, the disk drive has been provided with a clock and a counter for internally producing stepping pulses at the same rate as the external ones but with a phase difference of half the pulse period. The external and internal stepping pulses have been interleaved to provide a series of pulses with a repetition rate twice that of the external pulses and thereby to drive the stepper motor.
This conventional practice has proved disadvantageous when the clock used for generating the internal pulses is unpowered for saving power when the disk drive is standing by, that is, when no data transfer operation is in progress in the disk drive.
Among various power saving schemes heretofore suggested and used with disk drives is the one described and claimed in Tsuyuguchi et al. U.S. Pat. No. 4,658,307 assigned to the assignee of this application. Tsuyuguchi et al. teaches to connect the disk motor driver circuit, the stepper motor driver circuit, and the read/write circuit, all standard components of the disk drive, to a power supply via a power saving switch. This switch is closed only when a disk is loaded in the disk drive or is when the "drive select" and "motor on" signals from the host are in prescribed states. The motor driver circuits and read/write circuit are therefore not powered in the absence of a disk or when the "drive select" and "motor on" signals are in other than the prescribed states, even if the complete data processing system, comprising the host and one or more disk drives, is powered on. A very substantial saving of power can thus be accomplished.
It has also been suggested to reduce the waste of power by various other power consuming components in the disk drive. Such additional power consuming components include a supply voltage detector circuit for detecting whether the supply voltage has built up to a predetermined value when the system is switched on, a file protect sensor for sensing whether the loaded disk cartridge is protected against erasure or writing, a disk capacity sensor for discriminating between one-megabyte and two-megabyte disk cartridges which may be loaded interchangeably in the disk dive, and another disk capacity sensor for discriminating between four-megabyte and other capacity disks cartridges.
The present applicant has explored the possibilities of further reducing the power consumption of disk drives and manufactured an experimental disk drive in which the clock was unpowered when the disk drive was standing by. The trouble occurred because the clock as we have it today inherently remains unstable for some time after being powered on, failing to produce pulses at a constant repetition rate. Such irregular clock pulses during the startup period of the clock, conventionally used for production of internal stepping pulses, made it impossible in some cases to position the transducer on the desired track on the disk. It might be contemplated to circumvent this problem by making longer the spacings between the stepping pulses, but then the seek speed would drop correspondingly.